In a turbogenerator generating electricity and operating on a liquid fuel, it is necessary to increase the pressure of and atomize or vaporize the liquid fuel to be provided to the turbogenerator combustor. In addition, it is also desirable to increase the pressure of some of the turbogenerator compressor discharge air which is nominally supplied to the turbogenerator combustor and use this additionally compressed air to assist liquid fuel atomization in special fuel/air injectors used in the combustor. In order to have complete combustion, without the generation of undesirable combustion products such as COx and NOx, it is critical that the liquid fuel be completely atomized or vaporized when it enters the turbogenerator combustor. Further, if not fully atomized, the liquid fuel can leave varnish on any metal surfaces that it comes into contact with. The increased pressure liquid fuel and the increased pressure turbogenerator compressor discharge air (air assist air) can work together to accomplish complete atomization.
In addition, if the liquid fuel is at too high a temperature, the fuel injectors which deliver the liquid fuel to the turbogenerator can become vapor locked which will disrupt the continued flow of the liquid fuel to the combustor. It is, therefore, essential that the temperature of the liquid fuel be maintained below the temperature at which vapor lock can occur. Means to cool the liquid fuel may be required.
In a conventional turbogenerator operating on a liquid fuel, the speed of the turbogenerator is normally controlled by the interaction of liquid fuel flow rate and the load of the turbogenerator electrical output. Besides requiring a separate liquid fuel control and/or metering valve to regulate the liquid fuel flow rate, such a system requires a turbogenerator speed sensor, requires a turbogenerator turbine exhaust temperature sensor, is dependent upon turbogenerator load, would not be self-damping, and has certain inherent instabilities.
Further, in the operation of a turbogenerator, it has been difficult to sustain low power output operation. Inherently, the turbogenerator is designed for a continuous, steady-state, full power operation. When a low power output is required to be sustained, the fuel system does not inherently have the capability to adequately deal with this type of operation without some special measures being taken.
A new type of fuel pump and a new type of compressor to supply air assistance for fuel/air injectors appears to be warranted. Centrifugal pumps and compressors are potential candidates for both liquid fuel pressurization and control and for air compression used for fuel/air atomizing injectors. However, centrifugal pumps and compressors operate best (with high efficiencies) when they have a high throughput flow rate and a low pressure rise relative to their tip speed. These operating conditions are characterized as high specific-speed conditions. Under these conditions, a centrifugal compressor can operate with an efficiency on the order of seventy-eight percent (78%). But the flow rate and pressure rise requirements for fuel pressurization and air assist compression for the liquid fuel pressurization and control system are for low specific-speed compressors (low throughput flow rate and high pressure rise relative to the compressor's tip speed). A centrifugal pump and compressor operating under these conditions would have an efficiency of less than twenty percent (20%). Under these conditions it would require a very large number of centrifugal compressors in series (e.g. ten (10)) to produce the same pressure rise for a given tip speed as could one (1) helical flow compressor.
A helical flow compressor is a high-speed rotating machine that accomplishes compression by imparting a velocity head to each fluid particle as it passes through the machine's impeller blades and then converting that velocity head into a pressure head in a stator channel that functions as a vaneless diffuser. While in this respect a helical flow compressor has some characteristics in common with a centrifugal compressor, the primary flow in a helical flow compressor is peripheral and asymmetrical, while in a centrifugal compressor, the primary flow is radial and symmetrical. The fluid particles passing through a helical flow compressor travel around the periphery of the helical flow compressor impeller within a generally horseshoe shaped stator channel. Within this channel, the fluid particles travel along helical streamlines, the centerline of the helix coinciding with the center of the curved stator channel. This flow pattern causes each fluid particle to pass through the impeller blades or buckets many times while the fluid particles are traveling through the helical flow compressor, each time acquiring kinetic energy. After each pass through the impeller blades, the fluid particles reenter the adjacent stator channel where they convert their kinetic energy into potential energy and a resulting peripheral pressure gradient in the stator channel. The multiple passes through the impeller blades (regenerative flow pattern) allows a helical flow compressor to produce discharge heads of up to fifteen (15) times those produced by a centrifugal compressor operating at equal tip speeds. A helical flow compressor operating at low specific-speed and at its best flow can have efficiencies of about fifty-five percent (55%) with curved blades and can have efficiencies of about thirty-eight percent (38%) with straight radial blades.
A helical flow pump has the same basic design as a helical flow compressor.
Among the advantages of a helical flow pump or compressor or a helical flow turbine are:
(a) simple, reliable design with only one rotating assembly;
(b) stable, surge-free operation over a wide range of operating conditions (i.e. from fill flow to no flow);
(c) long life (e.g., 40,000 hours) limited mainly by their bearings;
(d) freedom from wear product and oil contamination since there are no rubbing or lubricated surfaces utilized;
(e) fewer stages required when compared to a centrifugal compressor; and
(f) higher operating efficiencies when compared to a very low specific-speed (high head pressure, low impeller speed, low flow) centrifugal compressor.
The flow in a helical flow pump or compressor can be visualized as two fluid streams which first merge and then divide as they pass through the pump or compressor. One fluid stream travels within the impeller buckets and endlessly circles the pump or compressor. The second fluid stream enters the pump or compressor radially through the inlet port and then moves into the horseshoe shaped stator channel which is adjacent to the impeller buckets. Here the fluids in the two streams merge and mix. The stator channel and impeller bucket streams continue to exchange fluid while the stator channel fluid stream is drawn around the pump or compressor by the impeller motion. When the stator channel fluid stream has traveled around most of the compressor periphery, its further circular travel is blocked by the stripper plate. The stator channel fluid stream then turns radially outward and exits from the compressor through the discharge port. The remaining impeller bucket fluid stream passes through the stripper plate within the buckets and merges with the fluid just entering the compressor/turbine.
The fluid in the impeller buckets of a helical flow pump or compressor travels around the compressor at a peripheral velocity which is essentially equal to the impeller blade velocity. It thus experiences a strong centrifugal force which tends to drive it radially outward, out of the buckets. The fluid in the adjacent stator channel travels at an average peripheral velocity of between five (5) and eighty (80) percent of the impeller blade velocity, depending upon the pump or compressor discharge flow. It thus experiences a centrifugal force which is much less than that experienced by the fluid in the impeller buckets. Since these two centrifugal forces oppose each other and are unequal, the fluid occupying the impeller buckets and the stator channel is driven into a circulating or regenerative flow, The fluid in the impeller buckets is driven radially outward and "upward" into the stator channel. The fluid in the stator channel is displaced and forced radially inward and "downward" into the impeller bucket.
While the fluid in either a helical flow pump or compressor is traveling regeneratively, it is also traveling peripherally around the stator-impeller channel. Thus, each fluid particle passing through a helical flow pump or compressor travels along a helical streamline, the centerline of the helix coinciding with the center of the generally horseshoe shaped stator-impeller channel.